The ads are not a waste. It's called remarketing and it does boost conversion numbers over raw untargetted ads. The ads are targeting people who went to a site, browsed around, didn't buy but might do in future (much more likely than a random unselected person).

Generally the ad network does not know if you really completed a purchase or dropped off during the funnel. Presumably advertisers could set up a hook to inform the ad network if you did purchase or not, but I guess they don't care enough to do so.